Typically, motion picture films released for public exhibition include four soundtracks each recorded in a different format. The four different format soundtracks collectively comprise a “quad” format optical soundtrack. The quad format advantageously allows reproduction by equipment compatible with any one of the four-recorded formats. The four separate audio tracks have different locations on the film. For example, the sound track for a Digital Theater System Corp. or DTS® formatted sound file lies between the edge of the film frame and the SMPTE standardized location for a variable area audio track. (DTS® is a registered mark of the Digital Theater System Corp.) The DTS® code track provides a synchronization signal for an external DTS® CD player which can provide six audio channels. A Dolby SR® encoded track lies in the variable area audio track position and this signal provides backwards compatibility for cinema sound processors incapable of signal decoding. (Dolby SR® is a registered mark of Dolby Laboratories Inc.) The Dolby SR® track offers the simplest reproducing system, namely a stereo formatted audio signal, or stereo plus two additional channels. A Dolby Digital® (SR.D) track lies in the area between film perforations and supports six channels of audio and is typically known as 5.1. A fourth recording format developed by Sony and known as Sony Dynamic Digital Sound® or SDDS® offers eight channels of audio with data recorded at the edges of the film. (Sony Dynamic Digital Sound® and SDDS® are registered marks of Sony Corp.) In this way, the quad format optical soundtrack offers enhanced playback capability that is backwardly compatible with stereo variable area (SVA) cinema sound processors.
To appreciate the composition of the quad format soundtrack, a consideration of the original sound assembly procedures will be helpful. A typical mixing operation to create a quad format sound track combines various separate sources, including dialogue, sound effects, ambiance, and music, with each originating from mono or multi-track sources. The mixing operation yields a six channel discrete sound format known as the original master mix. The term discrete sound format typically means that no relationship exists among different channels. The original master mix includes dialogue that represents about 95% of the normal sound content and is usually located in the center channel, sometimes 5% can be located or combined on left and/or right or surround channels and manipulated for effect, for example emanating from a radio, TV or telephone. If required, an operator can add reverberation on the center channel alone, or occasionally on the lateral or surround channel. Sound effects related to the dialogue, such as foot steps, etc exist with the dialogue in the center channel. Other effects can be located on the lateral or surround channels to increase the sound perspective. Special effects normally exist on all channels depending to the required result. Action ambiance is normally located in the lateral or surround channels. However, sometimes an operator will place the ambience in the center channel if such ambience exists as part of the original sound track. If an original music recording exists in a multi-channel format, the center channel will typically contain any solo instrument or vocalist. The lateral, surround and subwoofer channels provide the main support for the sound contents.
During mixing of various signals, audio processors can provide reverberation and can add perspective by the use of delay or special filter functions, simulation on music or ambiance. These acoustic enhancements, while fully permissible, nonetheless can introduce unexpected and undesired phase shifts, during encoding, such as by as Dolby SR® encoding, and subsequently reproduced in a monaural or a two-track stereo format.
In the digital film domain, the discrete tracks can very faithfully deliver to the listener the original sound perspective of the master mix. However, the various coding algorithms employed by the three digital systems can introduce differences into the sound. For example, one encoding system includes the subwoofer channel sound content in the surround channels, thus using just five tracks instead of six.
Mixing the original six channel master mix into a four track master mix yields an analog format audio signal comprising left, right, center and surround channels. These four channels are processed, for example using a Dolby SR® 4:2 spatial encoder, to form a two-track encoded audio signal which enables stereo reproduction and, in addition, also enables decoding to restore the four tracks of the master mix with substantially similar quality. The encoder output produces two encoded channels identified as Left total and Right total or Lt-Rt. These two encoded tracks pass through two Dolby SR® noise reduction processors for recording on the optical negative film. During film exhibition, these two encoded tracks are reproduced and coupled, for example, via and appropriate Dolby SR® equipped reader followed by a 2:4 decoder which transforms the encoded channels Lt-Rt to recreate the original four discrete channels, Left, Right, Center and Surround.
Ideally, the four soundtrack formats should be substantially similar in contents, if not identical, within the constraints of each individual system parameters. However, manipulation of various acoustic parameters in the digitally formatted tracks can produce unwanted and unexpected consequences, especially when using Dolby Surround encoding and decoding. Thus, a need exists need for a technique for rapidly identifying the occurrence of such unwanted acoustic consequences.